Effect of thermal cycles on microstructure and extrudability of AA6XXX alloys

The microstructural response of two aluminum extrusion alloys, 6063 and 6105, to thermal and mechanical simulations was studied so as to reveal the roles of temperature, mechanical deformation and alloy chemistry in the evolution of the microstructure during commercial extrusion operations. These alloys are age-hardenable, with magnesium silicide, Mg₂Si, being the main strengthening precipitate and the state of this precipitate being affected by various steps of the extrusion process. Simulations of various steps of the extrusion process from preheat to extrusion press exit were performed on the Gleeble 1500 thermo-mechanical testing machine. Preheat times and temperatures and extrusion temperatures were gathered from data during extrusion runs at Keymark Corporation, Fonda, NY, and entered into the laboratory simulations. A preheat temperature of 450°C and an extrusion temperature of 515°C was used in the simulations. Magnesium silicide content was examined using the scanning electron microscope and average particle size as well as weight percent calculations were made based on the observations. Hardness measurements were taken as a measure of the solution condition. The magnesium silicide was observed to precipitate or grow in the presence of extrusion temperatures and deformation in the 6105 alloy and only slightly decrease for the 6063 alloy. Deformation assisted diffusion and issues related to the location of the solvus on the classical phase diagram were used to explain the behavior of this alloy.; Studies were also conducted on as-cast material. Simulations performed on the as-cast 6063 alloy showed that micro-segregation profiles observed with the electron microprobe decreased slightly after the preheat simulations and decreased even more after the extrusion simulations. Transformation of β-AlFeSi to α-AlFeSi in the microstructure of the as-cast material was monitored using the electron microprobe for the two preheat simulations at 450°C and 510°C and the two extrusion simulations at 515°C and 570°C. Results showed that the transformation, which is reported to increase extrudability, did not occur significantly in the preheat simulations or extrusion simulations.; The thermal cycles do not fully dissolve the magnesium silicide and in some instances, such as in the case of the 6105 alloy, the combined effect of temperature and deformation actually caused an increase in the amount of magnesium silicide in the structure from 0.18 wt% to 0.27 wt%. Dissolution of magnesium silicide requires higher temperatures than indicated by the classical phase diagram; the classical phase diagram appears to be in error. Commercial preheating and extrusion thermal cycles were shown to reduce micro-segregation profiles in the as-cast material but lacked the necessary time to transform the β-AlFeSi to the α-AlFeSi.